The present invention relates to an SiO2—TiO2 glass suited especially for components used in EUV lithography, such as substrate materials for reflective mirror optics and masks or the like.
During a lithographic process, the structures for integrated circuits present on the mask are transferred to a silicon wafer by projection of laser radiation of a given wavelength. Especially in EUV lithography a wavelength of approximately 13 nm is used. Given the fact that there do not exist any materials that are pervious to light of that wavelength, reflective masks and optics are used in that process. It is the object of that technique to realize on the silicon wafer structures in widths of up to 35 nm.
SiO2—TiO2 glasses with a TiO2 content in the range of between approximately 6 and 8 percent by weight, for example, are employed as a preferred material in the production of components for EUV lithography, the thermal expansion occurring in the temperate range of between −50 and +100° Celsius being very small. For example, a glass of that type having a TiO2 content of 6.85 percent by weight shows zero expansion in the temperature interval from 19 to 25° Celsius.
Flame hydrolysis is a commonly used method for the production of SiO2—TiO2 glasses. As part of that method, gaseous SiO2 (for example SiCl4− or Si-alkoxide vapor) and TiO2 precursors (such as TiCl4− or Ti-alkoxide vapor) are exposed to a natural gas flame or a detonating gas flame (compare in this regard U.S. Pat. No. 5,970,751, WO 0232622 and U.S. Pat. No. 4,491,604, for example). The initial compounds thereby react, forming SiO2 and TiO2 droplets or mixtures thereof, which in turn are deposited on a die positioned below the flame. As a rule, the temperature conditions are selected to ensure that a compact glassy body is formed by that process. The process is also generally known as flame-hydrolytic direct deposition.
Flame-hydrolytic direct deposition is a preferred method for the production of SiO2—TiO2 glasses, being a single-step process by means of which relatively large dimensions (masses of up to several hundred kilograms) can be produced in a comparatively low-cost way.
During EUV lithography, the structures to be transferred from the mask are inscribed by an electron beam. The realization of structures of smaller widths requires in this case ever higher acceleration speeds. As a result, instead of being moderated by the layers near the mask surface, an ever greater part of the electron beam will penetrate into and damage the substrate material below those layers. That damage normally makes itself felt by compaction of the material in the irradiated places. As it is only the irradiated side of the substrate material that gets compacted, i.e. that shrinks, the substrate may get distorted. This is a critical factor with respect to the imaging quality. The specifications for EUV mask substrates prescribe a flatness value of 50 nm PV (peak-to-valley value according to SEMI P37-1101). Extensive polishing and finishing processes are necessary if this value is to be reached. Any subsequent variation, which may occur for example during electron beam irradiation while inscribing the mask, may become critical already at a distortion of a few 10 nm.
Now, it has been found that SiO2—TiO2 glasses produced by the flame-hydrolysis process are especially sensitive to damage by radiation.
In view of this it is a first object of the present invention to disclose an improved SiO2—TiO2 glass which, compared with conventional SiO2—TiO2 glasses, offers improved resistance to radiation.
It is a second object of the invention to disclose an improved SiO2—TiO2 glass which is suited in particular for use in EUV lithography.
It is a third object of the invention to disclose a manufacturing process for the production of an improved SiO2—TiO2 glass which, compared with conventional SiO2—TiO2 glasses, offers better resistance to radiation.